1. Field of the Invention
The present invention relates to normal and emergency lighting systems, and more particularly, to such lighting systems for operating high intensity discharge lamps.
2. Background Information
High intensity discharge (HID) lamps such as mercury, metal halide and high pressure sodium lamps, are often used to provide outdoor (and in some instances indoor) lighting because of their high efficiency. However, should the power normally supplied by the utility company fail, emergency lighting is often provided by a separate, auxiliary lighting system with its own power source. Such auxiliary lighting systems typically include a power source control circuit which detects the loss of AC power from the normal power source, and switches in a backup power source such as a set of batteries.
Emergency lighting is often not provided by the HID lamps themselves because the detection of the AC power failure and the subsequent transfer from the normal source to the backup source is typically not quick enough to maintain arc conductance in a hot HID lamp. Consequently, the HID lamp "goes out" upon the initial loss of power and usually cannot be restarted until the lamp has been allowed to cool. This cooling period can be as long as 15 minutes depending upon the lamp type, temperature, starting voltage and other factors. Thus it may not be possible to restore normal lighting until after the cool down period which may be much longer than the actual duration of the disruption of power.
Many emergency lighting systems have utilized incandescent lamps as backup lamps for the HID lamps. However, incandescent lamps generally are not as efficient as HID lamps and therefore can require a much larger battery capacity to match the illumination provided by comparable HID lamps.
Another lamp which can be used as a backup lamp for HID lamps is a fluorescent lamp. However, fluorescent lamps while more efficient, are discharge lamps and require their own ballasts. Ballast circuits for discharge lamps provide a high initial voltage potential to initiate arc conductance, and can add to the expense of the emergency lighting system.
Separate backup lamps can also require additional mounting fixtures which can be both expensive and unsightly. Alternatively, the backup lamps may be installed within the HID fixture, but such an arrangement often disrupts the light distribution of both the HID lamp and the backup lamp, and can cause higher operating temperatures.
To enable the use of the same HID lamps to provide both the normal and emergency lighting, some previous lighting systems have proposed the use of uninterruptable power supplies which maintain AC power regardless of the condition of the utility source. Such uninterruptable power supplies typically include a rectifier to convert the AC power from the utility source to DC power and a central inverter to reconvert the DC power back to AC power which is distributed to the ballasts for each individual HID lamp. Should power from the utility source fail, a battery supplies DC power to the central inverter so that the supply of AC power to the HID lamp ballast is not interrupted.
One disadvantage of such an arrangement is that if the central inverter fails, all of the HID lamps supplied by the central inverter fail as well, and the emergency lighting is lost. In addition, because the inverter usually operates at low frequency (typically 60 Hz), the inverter often requires large, heavy and expensive components. Likewise, the individual ballasts of the HID lamps also tend to be large and heavy as a result of the low frequency operation. Still further, low frequency operation of the inverter and the ballast can create significant and therefore distracting audible noise.
Still another disadvantage of such previous systems is that the HID lamps are typically operated at full power in the emergency mode. This can require additional battery capacity, and can create a high glare illumination pattern. Moreover, the overall system efficiency is often low.
As previously mentioned, discharge lamps typically have a ballast circuit between the power source and the lamp itself to provide a high initial voltage potential to start the lamp and further to limit the arc current of the lamp to a safe level. Furthermore, many ballast circuits are provided to regulate the lamp power to within standard limits (per ANSI C78.1350).
One prior ballast circuit for current limitation and power regulation includes an impedance in series with the lamp. This series impedance often takes the form of one or more passive reactive circuit elements such as an inductor. Such a prior ballast circuit can limit arc current to safe levels, but typically does not employ feedback to adjust the level of ballast impedance to regulate the lamp power against changes in the input line voltage which can often vary as much as .+-.10% from the nominal line voltage. Furthermore, these prior ballast circuits also frequently do not regulate lamp power against changes in the lamp operating voltage which can rise by a factor greater than 50% over the life of the lamp.
Other ballast circuits have been used which include a transformer in which the core saturation is controlled to offset variations in the input line voltage or lamp operating voltage. However, such ballast circuits have often been only partially effective in providing lamp power regulation.
As a result, power regulation is often relatively loose resulting in large variations in light output as the input voltage varies or as the lamp ages. Furthermore, the life of the lamp can be often degraded when operated at greater than the rated power level.
Still further, these passive reactor ballasts tend to be large, heavy and noisy due to their relatively low frequency of operation. Moreover, the light output often strobes at twice the line frequency of the power source, which can not only be distracting, but can also be dangerous if used to illuminate areas in which rotating machinery is operated.
Other ballast circuits have used hybrid combinations of electronic control circuitry and magnetic reactors to give tighter lamp power regulation through the use of feedback control. For example, current in an additional winding of the magnetic reactor core has been used to vary the effective impedance of the main lamp current paths.
These ballast circuits, like the passive reactor ballast, are also operated at relatively low frequency and therefore similarly tend to be large, heavy and noisy. Moreover, the regulating technique can create current harmonics in the power supply wires which may effect the operation of other devices supplied by the same power source. Furthermore, the overall power factor of the lighting system can be relatively low, requiring a higher current to be supplied.